Which term best describes "one-way" encryption that is not reversible?

The term that best describes "one-way" encryption that is not reversible is a hash. Hash functions take an input (or "message") and return a fixed-size string of characters, which is typically a digest that is unique to each unique input. This process is inherently one-way; once data is converted into a hash, it cannot be converted back to its original form. This characteristic makes hashes particularly useful for ensuring data integrity and for securely storing passwords.

Hashing is designed to be a one-directional operation where it's easy to compute a hash from input data, but it is computationally infeasible to revert the hash back to the original input. This property sets it apart from traditional encryption methods, which are reversible through decryption processes.

The other terms do not suitably describe this unique feature. Encryption refers generally to processes that make data unreadable to unauthorized users, while decryption is the process of converting encrypted data back into a readable format. Symmetric encryption involves using the same key for both encryption and decryption, making it a reversible process, unlike hashing.